TYP/DOC: importing from api.py/__init__.py

[twoertwein]

Two questions related to symbols exported in the various api.py files that currently come up in a few pandas-stubs PRs.

1. Are all symbols in the api.py files meant to be public (I assume yes, some might be missing in the docs)
2. Is it intended to import directly from those api.py files (I assume no, instead they should be imported from the pandas namespace)

Similar to api.py, users might want to import symbols from __init__.py files. I assume symbols in __init__.py might not necessarily be public and should not be important from there (except from the root __init__.py).

@Dr-Irv @bashtage @jreback